<meta charset="utf-8">
(#) Bidirectional text spoofing

!!! ERROR: Bidirectional text spoofing
   This is an error.

Id
:   `BidiSpoofing`
Summary
:   Bidirectional text spoofing
Severity
:   Error
Category
:   Security
Platform
:   Any
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   7.1.0 (January 2022)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://krebsonsecurity.com/2021/11/trojan-source-bug-threatens-the-security-of-all-code/
See
:   https://goo.gle/BidiSpoofing
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/BidirectionalTextDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/BidirectionalTextDetectorTest.kt)

Unicode bidirectional text characters can alter the order in which the
compiler processes tokens. However, this can also be used to hide
malicious code, and can be difficult to spot. This lint check audits the
source code and looks for cases where it looks like bidirectional text
has the potential to be misleading.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/StretchedString.java:5:Error: String contains misleading Unicode
bidirectional text [BidiSpoofing]
    if (accessLevel != "user‮ ⁦// Check if admin⁩ ⁦") {
                       -----------------------------
src/CommentingOut.java:5:Error: Comment contains misleading Unicode
bidirectional text [BidiSpoofing]
    /*‮ } ⁦if (isAdmin)⁩ ⁦ begin admins only */
    -------------------------------------------
src/CommentingOut.java:7:Error: Comment contains misleading Unicode
bidirectional text [BidiSpoofing]
    /* end admins only ‮ { ⁦*/
    --------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are the relevant source files:

`src/StretchedString.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
// From https://github.com/nickboucher/trojan-source/blob/main/Java/StretchedString.java
public class StretchedString {
    public static void main(String[] args) {
        String accessLevel = "user";
        if (accessLevel != "user‮ ⁦// Check if admin⁩ ⁦") {
            System.out.println("You are an admin.");
        }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`src/CommentingOut.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
// From https://github.com/nickboucher/trojan-source/blob/main/Java/CommentingOut.java
public class CommentingOut {
    public static void main(String[] args) {
        boolean isAdmin = false;
        /*‮ } ⁦if (isAdmin)⁩ ⁦ begin admins only */
            System.out.println("You are an admin.");
        /* end admins only ‮ { ⁦*/
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`src/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
/* Comment ‮ // OK
 * and ⁦ // OK
 */
val valid1 = "Left‮Right‬Left" // OK
val valid2 = "Left‮Right⁦Nested Left⁩‬Left" // OK
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/BidirectionalTextDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("BidiSpoofing")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("BidiSpoofing")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection BidiSpoofing
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="BidiSpoofing" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'BidiSpoofing'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore BidiSpoofing ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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